This invention relates to a cardiac pacemaker for delivering electrical stimuli to the heart for the purpose of stimulating the heart to react electrically and thus contract mechanically. A pacemaker consists of an electronic generator connected through an electrode to the ventricular myocardium of the heart; the electrode may be unipolar or bipolar.
Although fixed rate generators are known which provide electrical stimuli to the heart at fixed and preset intervals, the present invention is concerned with a pacemaker having a demand generator, which is sensitive to the electrical activity of the heart. The demand generator includes a timing circuit which, unless disabled, causes a stimulus to be delivered at the end of an escape interval. If there is no intrinsic, or naturally occurring, heart activity, the ventricles are electrically inactive and the demand generator acts as a fixed rate generator and delivers pacing stimuli to the heart. If, however, the generator senses an intrinsic heart activity during an escape interval, then the timing circuit is restarted without causing a stimulus, while, if no ventricular activity is sensed during the escape interval, a stimulus is delivered by the generator at the end of that interval and the timing circuit restarted. The initial escape interval and the subsequent pacing interval between consecutive stimuli may be identical, or different. Thus, a demand generator delivers stimuli designed to depolarise the heart only if the natural ventricular rate falls below a preset value corresponding to the escape interval.
With a healthy heart, the natural rate of ventricular activity responds to nervous and humoral stimuli; in the normal individual, exercise results in an increasing heart rate, which is accomplished by nervous and humoral stimuli increasing the rate at which the natural pacemaker of the heart--the sinus node--depolarises. A pacemaker having a demand generator as described above is incapable of responding to physiological conditions which, in the normal individual, would cause an increase in heart rate. While there have bee proposals to cause the pacemaker to react to the physiological conditions of the individual, e.g. atrial activity, tissue pH or respiratory rate, all those proposals have required detectors which are additional to the pacemaker and which sense the required condition.
The present invention is based on my realisation that the period of ventricular repolarisation--the interval between the onset of ventricular depolarisation (the QRS complex) and repolarisation (the T wave)--decreases with increase in heart rate, due to the action of hormones released into the blood stream with cardiac effects. In the present invention, the interval between a pacing stimulus delivered by a pacemaker generator and the evoked repolarisation sets the escape interval of the generator for the subsequent stimulus, and thus controls the heart rate.